The present invention relates to black magnetic iron oxide particles and a process for producing the black magnetic iron oxide particles, and more particularly, to black magnetic iron oxide particles not only having a sufficient blackness and excellent electrical properties, flowability and electrification property (charging property), but also exhibiting a good compatibility with binder resins such as aromatic vinyl-based resins, acrylic-based resins and copolymer resins thereof, and a good dispersibility in these resins for a magnetic toner, a process for producing the particles, and a magnetic toner using the black magnetic iron oxide particles.
Hitherto, as one of electrostatic latent image development methods, there is well known a development method using a so-called one component magnetic toner as a developer, which contains no carrier and comprises composite particles obtained by mixing and dispersing black magnetic iron oxide particles such as magnetite particles in resins.
With recent tendency toward high speed and high image quality in laser beam printers or digital copying machines, it has been strongly demanded to improve properties of a magnetic toner used therein as a developer. For this reason, the magnetic toner has been required to show not only an improved electrification property and a high flowability, but also stable charging amount and flowability even upon change in environmental conditions such as temperature or humidity.
Due to the fact that the electrification property of the magnetic toner has a close relationship to electrical properties of black magnetic iron oxide particles used therein, it has been strongly demanded to provide black magnetic iron oxide particles having excellent electrical characteristics.
Thus, it has been required that the properties of the black magnetic iron oxide particles are further improved in order to enhance the electrification property of the magnetic toner.
More specifically, in order to obtain a magnetic toner having an excellent electrification property, it is necessary that the black magnetic iron oxide particles used therein have a sufficient blackness, a higher dispersibility and more excellent electrical properties.
In xe2x80x9cPOWDER AND POWDER METALLURGYxe2x80x9d, vol. 26, No. 7, pp. 239-240, it is described that xe2x80x9cthe degree of blackness of the specimen varies depending upon the content of Fe(II) and the average particle diameter, and particles having an average particle diameter of 0.2 xcexcm are bluish black and, therefore, are most suitable as black pigments . . . when the content of Fe(II) is not less than 10%, all specimens exhibit a black color though the degrees of blackness thereof are slightly different from each other, and when the content of Fe(II) is reduced to less than 10%, the color of the specimen is changed from black to reddish brownxe2x80x9d. As described in the above literature, it is known that the properties of black magnetic iron oxide particles for magnetic toner having a particle diameter of about 0.1 to about 0.5 xcexcm, varies depending upon mainly the content of Fe2+ (FeO). Thus, the black magnetic iron oxide particles have been required to have a large FeO content and a high blackness.
The dispersibility of the black magnetic iron oxide particles largely depends upon the surface conditions thereof. Therefore, in order to improve the surface conditions of the black magnetic iron oxide particles and to enhance the dispersibility thereof, it has been attempted to coat surfaces of the black magnetic iron oxide particles with a silicon compound or an aluminum compound. In addition, the black magnetic iron oxide particles tend to be magnetically agglomerated due to fineness thereof, resulting in the deterioration of the blending property with resins. Consequently, it has been required to prevent the black magnetic iron oxide particles from being magnetically agglomerated.
Also, the electrification property of the magnetic toner largely depends upon the surface conditions of the black magnetic iron oxide particles exposed to the surface of the magnetic toner. In particular, FeO contained in the black magnetic iron oxide particles acts for reducing an electrical resistance of the magnetic toner, so that the electrification property of the magnetic toner is considerably influenced by the content of FeO and the distribution of FeO in each particle. In this regard, Japanese Patent Application Laid-Open (KOKAI) No. 4-338971 describes that xe2x80x9cthe distribution condition of Fe(II) in the surface layer of magnetic iron oxide more highly contributes to stable frictional electrification property of the obtained toner under various environmental conditions rather than the FeO content thereinxe2x80x9d.
As described above, in order to enhance the dispersibility and electrical properties of the black magnetic iron oxide particles, it is necessary to optimize the FeO content and the abundance of FeO in the surface layer of each black magnetic iron oxide particle.
The magnetic toner has been produced by mixing the magnetic particles with aromatic vinyl resins such as styrene resins or vinyl toluene resins, acryl-based resins such as acrylic resins or methacrylic resins, and copolymer resins thereof. Therefore, the magnetic particles have been required to have an excellent dispersibility in these resins.
In the production of the magnetic toner, when the magnetic particles are kneaded with resins, it is difficult to sufficiently disperse the magnetic particles in resins since the magnetic particles have a hydrophilic surface, so that the magnetic particles are present in the form of agglomerated particles in resins after the kneading. When the non-uniform mixture is pulverized into such a size as required for a magnetic toner, there arise problems such as generation of a magnetic powder from agglomerated particles upon the pulverization or different contents of the magnetic particles in individual toner particles, thereby failing to obtain particles having excellent properties as a magnetic toner.
Consequently, in order to improve the surface conditions of the magnetic particles and to enhance the dispersibility thereof, it has been attempted to coat the surfaces of the magnetic particles with various coupling agents such as silane-based coupling agents and titanium-based coupling agents, or various metal compounds such as aluminum compounds and silicon compounds. Also, the magnetic particles are fine particles and, therefore, tend to be magnetically agglomerated together, resulting in deteriorated blending property with resins. Therefore, it has been required to inhibit the magnetic particles from being magnetically agglomerated together.
The flowability of the magnetic toner largely depends upon the surface conditions of the magnetic iron oxide particles exposed to the surface of the magnetic toner. Therefore, it has been required that the magnetic iron oxide particles themselves show an excellent flowability.
The magnetic toner tends to suffer from inherent drawbacks such as deteriorated flowability due to high resistance thereof. For this reason, it has been attempted to modify the surface of the magnetic toner or the surfaces of the magnetic particles contained in the magnetic toner, or enhance the flowability of the magnetic toner by adding external additives into the magnetic toner.
The magnetic toner is required to exhibit stable properties even upon any change in environmental conditions, for example, under high temperature and high humidity conditions or under low temperature and low humidity conditions. Thus, it has been strongly demanded to provide a magnetic toner capable of constantly showing stable flowability and charging amount.
Conventionally, it has been attempted to improve the dispersibility and electrical properties of the black magnetic iron oxide particles by limiting the FeO content thereof to a specific range (Japanese Patent Application Laid-Open (KOKAI) Nos. 58-189646(1983), 3-201509(1991), 4-338971(1992), 4-141664(1992) and 11-30877(1999)).
At present, it has been strongly demanded to provide black magnetic iron oxide particles for a magnetic toner, which can show not only a sufficient blackness but also excellent dispersibility and electrical properties. However, black magnetic iron oxide particles capable of fulfilling all of these properties simultaneously cannot be obtained until now. In particular, it has been most strongly demanded to provide black magnetic iron oxide particles for a magnetic toner which can show a sufficient blackness and excellent electrical properties as well as a good compatibility with binder resins such as aromatic vinyl-based resins, acrylic-based resins or copolymer thereof, and a good dispersibility in these magnetic toner resins. However, black magnetic iron oxide particles capable of fulfilling all of these properties simultaneously cannot be obtained until now.
That is, the conventional methods described in the above prior arts are classified into a method of production of particles in which the FeO content in whole black magnetic iron oxide particles is specified (Japanese Patent Application Laid-Open (KOKAI) Nos. 58-189646(1983), 3-201509(1991) and 11-30877(1999)), and a method of production of particles in which the FeO content is increased from an outer surface of each particle toward a center thereof (Japanese Patent Application Laid-Open (KOKAI) Nos. 4-338971(1992) and 4-141664(1992)). However, in any of these methods, the FeO content in a surface layer of each black magnetic iron oxide particle is large, so that the black magnetic iron oxide particles are unsatisfactory in electrical properties.
Further, in the methods described in the above prior arts, the FeO content is adjusted to the specified range by subjecting the black magnetic iron oxide particles to dry treatment under controlled conditions of dry atmosphere and drying temperature during the production process therefor. However, these methods fail to obtain such black magnetic iron oxide particles having a less FeO content only in a surface layer thereof.
Also, it has been attempted to enhance the flowability of the black magnetic iron oxide particles by using conventional surface treatments. However, the attempt also fails to obtain such iron oxide particles for a magnetic toner having a less FeO content only in the surface layer thereof and exhibiting an excellent flowability.
Consequently, the technical subject to be solved by the present invention aims at not only improving a dispersibility and electrical properties of the black magnetic iron oxide particles, but also especially enhancing a flowability thereof.
As a result of the present inventors"" earnest studies, it has been found that by (a) passing an oxygen-containing gas through an aqueous ferrous salt reaction solution containing ferrous hydroxide colloid obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution having an alkali concentration of less than one equivalent based on Fe2+ contained in the said aqueous ferrous salt solution, to produce black spinel-type iron oxide particles as core particles; (b) successively passing an oxygen-containing gas through a reaction solution having a pH of 4.0 to 5.0 and containing the obtained black spinel-type iron oxide particles as core particles, to surface-oxidize the said core particles; (c) adding an aqueous alkali hydroxide solution to the reaction solution obtained after the surface-oxidation thereof, to adjust the pH of the said reaction solution to 7.0 to 8.0; (d) passing an oxygen-containing gas through the obtained reaction solution to form a surface coat comprising fine spinel-type iron oxide particles on the surface of each core particle; and (e) successively passing an oxygen-containing gas through the resultant reaction solution, to oxidize the said surface coat comprising fine spinel-type iron oxide particles,
the obtained black magnetic iron oxide particles have not only a sufficient blackness and excellent electrical properties, but also excellent flowability and electrification property and a good dispersibility. The present invention has been attained based on the finding.
It is an object of the present invention to provide black magnetic iron oxide particles for a magnetic toner, which have a sufficient blackness and are excellent in dispersibility and electrical properties.
It is an object of the present invention to provide black magnetic iron oxide particles for a magnetic toner which not only have a sufficient blackness and excellent electrical properties, flowability and electrification property (charging property), but also exhibit a good compatibility with binder resins such as aromatic vinyl-based resins, acrylic-based resins and copolymer resins thereof, and a good dispersibility in these resins for a magnetic toner.
It is an object of the present invention to provide a magnetic toner which not only exhibits excellent electrification property and flowability, but also maintains the excellent electrification property and flowability stably for a long period of time.
To accomplish the aims, in a first aspect of the present invention, there are black magnetic iron oxide particles having an average particle diameter of 0.1 to 0.3 xcexcm, comprising:
black spinel iron oxide particles as core particles; and
a surface coat formed on the surface of each core particle, comprising fine spinel iron oxide particles, the content of FeO in a surface layer having a thickness of from a surface of each black magnetic iron oxide particle to about 3.5% per a radius of each black magnetic iron oxide particle, being 8 to 14% by weight based on the weight of Fe contained in said surface layer.
In a second aspect of the present invention, there are black magnetic iron oxide particles having an average particle diameter of 0.1 to 0.3 xcexcm, comprising:
black spinel iron oxide particles as core particles;
a surface coat formed on the surface of each core particle, comprising fine spinel iron oxide particles; and
a coating formed on the surface coat, comprising an organic compound having a hydrophobic group,
the content of FeO in a surface layer having a thickness of from a surface of each black magnetic iron oxide particle to about 3.5% per a radius of each black magnetic iron oxide particle, being 8 to 14% by weight based on the weight of Fe contained in said surface layer.
In a third aspect of the present invention, there are black magnetic iron oxide particles having an average particle diameter of 0.1 to 0.3 xcexcm , comprising:
black spinel iron oxide particles as core particles;
a surface coat formed on the surface of each core particle, comprising fine spinel iron oxide particles; and
a coating formed on the surface coat, comprising an aluminum compound and/or a silicon compound,
the content of FeO in a surface layer having a thickness of from a surface of each black magnetic iron oxide particle to about 3.5% per a radius of each black magnetic iron oxide particle, being 8 to 14% by weight based on the weight of Fe contained in said surface layer.
In a fourth aspect of the present invention, there are black magnetic iron oxide particles having an average particle diameter of 0.1 to 0.3 xcexcm, comprising:
black spinel iron oxide particles as core particles;
a surface coat formed on the surface of each core particle, comprising fine spinel iron oxide particles; and
a coating formed on the ssurface coat, comprising fine particles of an oxide of at least one element selected from the group consisting of Al, Si, Zr and Ti,
the content of FeO in a surface layer having a thickness of from a surface of each black magnetic iron oxide particle to about 3.5% per a radius of each black magnetic iron oxide particle, being 8 to 14% by weight based on the weight of Fe contained in said surface layer.
In a fifth aspect of the present invention, there is a process for producing the black magnetic iron oxide particles as defined in the first aspect, which process comprises:
(a) passing an oxygen-containing gas through an aqueous ferrous salt reaction solution containing ferrous hydroxide colloid obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution having an alkali concentration of less than one equivalent based on Fe2+ contained in said aqueous ferrous salt solution, to produce a reaction solution having a pH of 4.0 to 5.0 and containing black spinel iron oxide particles as core particles;
(b) successively passing an oxygen-containing gas through said reaction solution to surface-oxidize said core particles;
(c) adding an aqueous alkali hydroxide solution to the reaction solution obtained after the surface-oxidation of the core particles to adjust the pH of said reaction solution to 7.0 to 8.0;
(d) passing an oxygen-containing gas through the resultant reaction solution, to form a surface coat comprising fine spinel iron oxide particles on the surface of each core particle; and
(e) successively passing an oxygen-containing gas through the reaction solution to oxidize said surface coat comprising fine spinel iron oxide particles.
In a sixth aspect of the present invention, there is a magnetic toner comprising the black magnetic iron oxide particles as defined in the first aspect.